Question

The angle between A=3i+4j+5k and B=3i+4j-5k is

Answer 1

Here, A.B=0 Hence angle b/w A and B is 90°.Hope this helps you.

Answer 2

Dot product of vector A and B is
A · B = |A||B| cosθ
Here θ = angle between vectors A and B

A = 3i + 4j + 5k
B = 3i + 4j - 5k

Dot product of given vectors
A · B = (3i + 4j + 5k) · (3i + 4j - 5k)
= 9 + 16 - 25
= 0

0 = |A||B| cosθ
cosθ = 0
cosθ = cos90
θ = 90°

∴ Angle between them is 90°